Mongoose throwing You must specify a field error while using a variable at place of an explicit string

My Mongoose Command

    Modelname.update(
        {$and :[ {'field1' : field1}, {'field2' : field2}, {'field3' : field3}, {'field4' : field4}]},
        {
            $set : {'timestamp' : timestamp},
            $inc : {speed_string: 1}
        },{upsert: true}, function (err, record)
        {
            if (err)
            {

            }
            else
            {

            }
        });

As you can see that speed_string is a variable which is supposed to get increased in the real document or get upserted . I have logged it just before the query and it is indeed a string (and not empty like the error reads) but the query is not going through and I am getting this error.

"errmsg":"'$inc' is empty. You must specify a field like so: {$inc: {: ...}}"}

When I am replacing that variable with 'expected_string' things are working fine and that is more confusing, nevertheless.

There is something I am doing wrong, help is appreciated.

via Gandalf the White